Motor drive control circuit and motor drive apparatus

ABSTRACT

Conventional position-sensorless motor drive control circuits start a motor by a method that has the disadvantage that the motor is not always started in the forward direction or by a method that has the disadvantage that a current detection resistor produces a power loss. According to the invention, a motor drive control circuit that controls, with drive signals, a drive portion that feeds drive currents to the stator coils of a motor has a drive signal output circuit that outputs rotor position detection drive signals to the drive portion before the starting of the motor and a detection circuit that receives the common-terminal voltage of the stator coils when the drive signal output circuit is outputting the rotor position detection drive signals and that detects, according to the common-terminal voltage of the stator coils, the position of the rotor before the starting of the motor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a motor drive control circuit ofa position-sensorless type, and to a motor drive apparatus provided withsuch a motor drive control circuit.

[0003] 2. Description of the Prior Art

[0004] In conventional position-sensorless motor drive control circuits,one way to start a motor is by forcibly vibrating the motor with apredetermined sequence that is unrelated to the position of the rotor(hereinafter refereed to as “Method I”). Another way is by providing acurrent detection resistor and feeding the stator coils of the motorwith current pulses before starting the motor so as to detect theenergization status of the stator coils with the current detectionresistor and then determine, according to the result of the detection,the starting logic state that permits the motor to be started in theforward direction (hereinafter refereed to as “Method II”).

[0005]FIG. 11 shows an example of the configuration of aposition-sensorless motor drive apparatus that starts a motor by MethodII described above. A drive portion 1 converts a drive voltage V_(D)into three-phase voltages, and outputs them to a three-phase brushlessmotor 2 (hereinafter referred to as the motor 2). A current detectionresistor R5 detects the energization status of the drive portion 1. Amotor drive control circuit 3′ detects the current that flows throughthe current detection resistor R5 before the motor 2 is started, thendetermines, according to the detected current, the starting logic statethat permits the motor to be started in the forward direction, and thencontrols the drive portion 1 with drive signals generated on the basisof the starting logic state.

[0006] However, Method I described above has the disadvantage that,since the motor is fed with three-phase voltages generated on the basisof a starting logic state that is unrelated to the rotor position, themotor is not always started in the forward direction. On the other hand,Method II has the disadvantage that the current detection resistorproduces a power loss. Thus, there have conventionally been available nomotor derive apparatuses that are suitable for use in portableappliances, such as portable MD (minidisk) players, in which rotation ofa disk is frequently started and stopped.

[0007] Incidentally, in the motor drive apparatus disclosed in JapanesePatent Application Laid-Open No. H5-268791, the rotor position isdetected by monitoring zero-cross voltages in coils that are in afloating state. With this detection method, however, it is not possibleto detect the rotor position unless the motor is rotating. Thus, withthis detection method, it is possible to detect the rotor position whensynchronism is recovered after being momentarily lost, but it isimpossible to detect the rotor position before the starting of themotor.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a motor drivecontrol circuit that operates with stable starting characteristics andwith low power consumption, a motor drive apparatus employing such amotor drive control circuit, and an electric appliance employing such amotor drive apparatus.

[0009] To achieve the above object, according to one aspect of thepresent invention, a motor drive control circuit that controls, withdrive signals, a drive portion that feeds drive currents to the statorcoils of a motor is provided with: a drive signal output circuit thatoutputs rotor position detection drive signals to the drive portionbefore the starting of the motor; and a detection circuit that receivesthe common-terminal voltage of the stator coils when the drive signaloutput circuit is outputting the rotor position detection drive signalsand that detects, according to the common-terminal voltage of the statorcoils, the position of the rotor before the starting of the motor.

[0010] According to another aspect of the present invention, a motordrive apparatus is provided with: a motor; a drive portion that feedsdrive currents to the stator coils of the motor; and a motor drivecontrol circuit configured as described above which controls the driveportion with drive signals.

[0011] According to still another aspect of the present invention, anelectric appliance is provided with: a motor drive apparatus configuredas described above; and a rotating member that is driven by the motordrive apparatus

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] This and other objects and features of the present invention willbecome clear from the following description, taken in conjunction withthe preferred embodiments with reference to the accompanying drawings inwhich:

[0013]FIG. 1 is a diagram showing the configuration of a motor driveapparatus embodying the invention;

[0014]FIG. 2 is a time chart of the signals that act as the rotorposition detection drive signals before the starting of the motor;

[0015]FIG. 3 is a time chart of the motor voltages when the rotorposition is detected before the starting of the motor;

[0016]FIG. 4 is a diagram showing the relationship between the registervalues stored in the register provided in the motor drive apparatusshown in FIG. 1 and the rotor position before the starting of the motor;

[0017]FIG. 5 is a diagram showing the relationship between the startinglogic state output from the decoder provided in the motor driveapparatus shown in FIG. 1 and the rotor position before the starting ofthe motor;

[0018]FIG. 6 is a diagram showing an example of the configuration of thecoil common-terminal variation detection comparator and the detectionlevel generation circuit provided in the motor drive apparatus shown inFIG. 1;

[0019]FIG. 7 is a diagram showing another example of the configurationof the coil common-terminal variation detection comparator and thedetection level generation circuit provided in the motor drive apparatusshown in FIG. 1;

[0020]FIG. 8 is a diagram showing the relationship between thecommon-terminal voltage of the stator coils and the rotor positionbefore the starting of the motor;

[0021]FIG. 9A is a time chart of the waveforms of the pulse signalsoutput from the pulse generator provided in the motor drive apparatusshown in FIG. 1 and the waveforms of the voltages observed at relevantpoints in the circuit shown in FIG. 6, in a case where the rotorposition before the starting of the motor is in the range of electricalangles from 240° to 270°;

[0022]FIG. 9B is a time chart of the waveforms of the pulse signalsoutput from the pulse generator provided in the motor drive apparatusshown in FIG. 1 and the waveforms of the voltages observed at relevantpoints in the circuit shown in FIG. 6, in a case where the rotorposition before the starting of the motor is in the range of electricalangles from 0° to 30°;

[0023]FIG. 10 is a diagram showing the configuration of an optical diskplayback apparatus embodying the invention; and

[0024]FIG. 11 is a diagram showing an example of the configuration of aconventional position-sensorless motor drive apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025]FIG. 1 shows an example of the configuration of a motor driveapparatus embodying the invention. Here, such circuit blocks andcomponents as are also found in FIG. 11 are identified with the samereference numerals and symbols. The motor drive apparatus shown in FIG.1 is designed for a three-phase motor, and is composed of a driveportion 1, a motor 2, and a motor drive control circuit 3.

[0026] The drive portion 1 is a three-phase drive circuit, and iscomposed of power transistors Q1 to Q6, which are n-channel MOSFETs(metal-oxide semiconductor field-effect transistors). The drains of thepower transistors Q1 to Q3 are connected together, and are connected toa terminal to which a drive voltage V_(D) is applied. The source of thepower transistor Q1 is connected to the drain of the power transistorQ4, the source of the power transistor Q2 is connected to the drain ofthe power transistor Q5, and the source of the power transistor Q3 isconnected to the drain of the power transistor Q6. The sources of thepower transistors Q4 to Q6 are connected together, and are connected toground.

[0027] The motor 2 has three stator coils Lu, Lv, and Lw. One end of thestator coil Lu is connected to the node between the power transistors Q1and Q4, one end of the stator coil Lv is connected to the node betweenthe power transistors Q2 and Q5, and one end of the stator coil Lw isconnected to the node between the power transistors Q3 and Q6. The otherends of the stator coils Lu, Lv, and Lw of the motor 2 are connectedtogether.

[0028] The motor drive control circuit 3 is connected to the individualnodes between the drive portion 1 and the motor 2, is also connected tothe other ends (connected together) of the stator coils Lu, Lv, and Lwof the motor 2, and is also connected, through terminals by way of whichto output drive signals D₁ to D₆, to the individual gates of the powertransistors Q1 to Q6 provided in the drive portion 1.

[0029] The drive portion 1 is controlled by the drive signals D₁ to D₆output from the motor drive control circuit 3. Then, the drive portion 1feeds drive currents to the motor 2 to rotate it.

[0030] Next, the motor drive control circuit 3 will be described. Themotor drive control circuit 3 is composed of a pulse generator 4, asequence circuit 5, a mode selector circuit 6, a coil common-terminalvariation detection comparator 7, a detection level generation circuit8, a register 9, a decoder 10, a preset circuit 11, a back electromotiveforce detection comparator 12, a switching noise mask circuit 13, and adrive waveform generation circuit 14. This configuration eliminates theneed to provide a current detection resistor for detecting theconduction status of the stator coils of the motor, and thus helpsreduce the power loss in the current detection resistor.

[0031] The pulse generator 4, on receiving an external signal (motorstart request signal) from an unillustrated control circuit or the like,outputs a short-period pulse signal T₁ (for example, a pulse signal with50 kHz pulses) for a predetermined period to the sequence circuit 5 andto the register 9.

[0032] The sequence circuit 5 operates only when it is receiving theshort-period pulse signal T₁. During this period, on the basis of theshort-period pulse signal T₁, the sequence circuit 5 generates signalsS₁ to S₆ and a period switch timing signal T₂ that indicates time pointst₁ , t₂, . . . , and t₆ (all these signals and time points are shown ina time chart in FIG. 2), and outputs the signals S₁ to S₆ and a startsignal K to the mode selector circuit 6.

[0033] The signals S₁ to S₆ are output only for so short a period as notto cause the motor to start to rotate. In this embodiment, the periodsfrom t₀ to t₁, from t₁ to t₂, . . . , and from t₅ to t₆ are each 600 μs.The signals S₁ to S₆ each have a high-level period, during which itremains high, a low-level period, during which it remains low, and apulse period, during which it has the same waveform as the short-periodpulse signal T₁. For example, the signal S₁ has a low-level periodduring the period from t₀ to t₂, a high-level period during the periodfrom t₂ to t₃, a pulse period during the period from t₃ to t₄, ahigh-level period during the period from t₄ to t₅, and a low-levelperiod during the period from t₅ to t₆,

[0034] The mode selector circuit 6, when it is receiving the startsignal K, operates in a rotor position detection mode; specifically, itselects the signals S₁ to S₆ and outputs them as the drive signals D₁ toD₆ to the drive portion 1 in order to turn on and off the powertransistors Q1 to Q6 provided in the drive portion 1. Specifically,during the period from t₀ to t₁, the power transistors Q1, Q5, and Q6are off, the power transistors Q2 and Q3 are on, and the powertransistor Q4 performs switching operation; during the period from t₁ tot₂, the power transistors Q1, Q2, and Q6 are off, the power transistorsQ4 and Q5 are on, and the power transistor Q3 performs switchingoperation; during the period from t₂ to t₃, the power transistors Q2,Q4, and Q6 are off, the power transistors Q1 and Q3 are on, and thepower transistor Q5 performs switching operation. Likewise, during theperiod from t₃ to t₄, the power transistors Q2, Q3, and Q4 are off, thepower transistors Q5 and Q6 are on, and the power transistor Q1 performsswitching operation; during the period from t₄ to t₅, the powertransistors Q3, Q4, and Q5 are off, the power transistors Q1 and Q2 areon, and the power transistor Q6 performs switching operation; during theperiod from t₅ to t₆, the power transistors Q1, Q3, and Q5 are off, thepower transistors Q4 and Q6 are on, and the power transistor Q2 performsswitching operation. Accordingly, during the period from t₀ to t₆, themotor voltages V_(uout), V_(vout), and V_(wout), i.e., the voltages atthe individual nodes between the drive portion 1 and the motor 2, havewaveforms as shown in a time chart in FIG. 3. In this way, short-periodcurrent pulses having the same period as the short-period pulse signalT₁ are fed to one of the stator coils Lu, Lv, and Lw at a time.

[0035] The rotor of the motor 2 is fitted with a magnet, and themagnetic field produced by this magnet causes the impedances of thestator coils to vary. Thus, the impedances of the stator coils vary withthe position of the rotor. Accordingly, when short-period current pulsesare fed to the stator coils, their common-terminal voltage CT varieswith the rotor position. Incidentally, the aim of giving a short periodto the current pulses fed to the stator coils is to increase theimpedances of the stator coils and thereby increase the variation widthof their common-terminal voltage CT.

[0036] The coil common-terminal variation detection comparator 7compares the common-terminal voltage CT of the stator coils with adetection level output from the detection level generation circuit 8,and outputs their differential signal Co to the register 9.

[0037] The detection level generation circuit 8 receives the motorvoltages V_(uout), V_(vout), and V_(wout) and the period switch timingsignal T₂ output from the sequence circuit 5, and generates differentdetection levels corresponding to the different periods (i.e., theperiods from t₀ to t₁, from t₁ to t₂, . . . , and from t₅ to t₆).Incidentally, how many times to feed the short-period current pulses,the frequency of the short-period current pulses, and the detectionlevels are determined to suit the motor 2.

[0038] The register 9 receives the short-period pulse signal T₁, andlatches the differential signal Co on every trailing edge of theshort-period pulse signal T₁. Moreover, the register 9 receives theperiod switch timing signal T₂, and successively stores, as resistorvalues R₁ to R₆, the differential signal Co latched at the time pointst₁ to t₆, respectively. The register 9 then outputs the register valuesR₁ to R₆ to the decoder 10. Incidentally, if the differential signal Cois high when stored, the value “1” is stored as the register valuecorresponding to that time point, and, if the differential signal Co islow when stored, the value “0” is stored as the register valuecorresponding to that time point.

[0039] In this embodiment, the relationship between the resistor valuesR₁ to R₆ and the rotor position before the starting of the motor is asshown in FIG. 4. In FIG. 4, rotor positions “1” to “12” correspond tothe range of electrical angles from 0° to 30°, from 30° to 60°, . . . ,and from 330° to 360°, respectively. In the decoder 10 is previouslystored the relationship shown in FIG. 4 between the resistor values R₁to R₆ and the rotor position before the starting of the motor, so thatthe decoder 10 detects, on the basis of the register values R₁ to R₆output from the register 9, the rotor position before the starting ofthe motor. Also stored previously in the decoder 10 are differentstarting logic states for different rotor positions as shown in FIG. 5,so that the decoder 10 feeds the preset circuit 11 with a starting logicstate, in the form of a combination of individual logic states Y₁ to Y₃,that corresponds to the rotor position detected before the starting ofthe motor. In FIG. 5, rotor positions “1” to “12” are the same as thoseshown in FIG. 4; moreover, “H” demands that the corresponding motorvoltage be in a high-level (=the voltage V_(D)) period, “M” demands thatthe corresponding motor voltage be in a high-impedance period, and “L”demands that the corresponding motor voltage be in a low-level (=theground potential) period; furthermore, the individual logic states Y₁,Y₂, and Y₃ indicate the logic states of the individual motor voltagesV_(uout), V_(vout), and V_(wout), respectively.

[0040] The sequence circuit 5, on completion of its operation sequence,feeds a preset signal PR to the preset circuit 11. On receiving thepreset signal PR, the preset circuit 11 feeds the starting logic stateY₁ to Y₃ to the drive waveform generation circuit 14.

[0041] According to the starting logic state Y₁ to Y₃, the drivewaveform generation circuit 14 generates signals MTX1 to MTX6, and feedsthem to the mode selector circuit 6. The mode selector circuit 6, whenit ceases to receive the start signal K from the sequence circuit 5,starts to operate in a sensorless drive mode; specifically, it selectsthe signals MTX1 to MTX6 and feeds them as the drive signals D₁ to D₆ tothe drive portion 1.

[0042] The operation described above makes it possible to surely startthe motor in the forward direction. After the motor is started, the backelectromotive force detection comparator 12 detects, on the basis of themotor voltages V_(uout), V_(vout), and V_(wout) and the common-terminalvoltage CT of the stator coils, the back electromotive forces appearingin the stator coils in a floating state, and outputs a detection signalof those back electromotive forces to the switching noise mask circuit13. On receiving the detection signal of the back electromotive forces,the switching noise mask circuit 13 removes therefrom switching noisegenerated by the switching operation of the power transistors Q1 to Q6,and then outputs it to the drive waveform generation circuit 14. Thedrive waveform generation circuit 14 has already detected, on the basisof the detection signal of the back electromotive forces, the rotorposition during the rotation of the motor, and generates the signalsMTX1 to MTX6 according to the rotor position. Here, the mode selectorcircuit 6 is operating in the sensorless drive mode, and therefore itoutputs the signals MTX1 to MTX6 as the drive signals D₁ to D₆ to thedrive portion 1.

[0043]FIG. 6 shows an example of the configuration of the coilcommon-terminal variation detection comparator 7 and the detection levelgeneration circuit 8. The motor voltage V_(vout) is fed to one end of aresistor R1, the motor voltage V_(vout) is fed to one end of a resistorR2, and the motor voltage V_(wout) is fed to one end of a resistor R3.The other ends of the resistors R1 to R3 are connected together, and areconnected to the non-inverting input terminal of a differentialamplifier 15. The output terminal and inverting input terminal of thedifferential amplifier 15 are connected to one end of a resistor R4. Aterminal to which a constant voltage V_(CC) is applied is connectedthrough a constant current source 16 to the other end of the resistor R4and to the inverting input terminal of a comparator 18, and then througha constant current source 17 to ground. The comparator 18 receives, atits non-inverting input terminal, the common-terminal voltage CT of thestator coils. The comparator 18 outputs a differential signal Co.

[0044] The circuit configured as described above operates in thefollowing manner. The resistors R1 to R3 produces a virtualcommon-terminal voltage from the motor voltages V_(uout), V_(vout), andV_(wout). The differential amplifier 15 buffers the virtualcommon-terminal voltage.

[0045] The resistor R4, the constant current source 16, and the constantcurrent source 17 produce a detection level by offsetting the virtualcommon-terminal voltage. The constant current sources 16 and 17 areturned on and off by a signal T₂′ based on the period switch timingsignal T₂. During the periods from t₀ to t₁, from t₂ to t₃, and from t₄to t₅, the constant current source 16 is off, and the constant currentsource 17 is on. This causes a current I₂ to be extracted from theresistor R4, and thus the level of the common-terminal voltage isshifted downward. Incidentally, in this embodiment, the level is shiftedby −40 mV. On the other hand, during the periods from t₁ to t₂, from t₃to t₄, and from t₅ to t₆, the constant current source 16 is on, and theconstant current source 17 is off. This causes a current I₁ to besupplied to the resistor R4, and thus the level of the common-terminalvoltage is shifted upward. Incidentally, in this embodiment, the levelis shifted by +40 mV.

[0046] The comparator 18 corresponds to the coil common-terminalvariation detection comparator 7 shown in FIG. 1. The comparator 18,when the motor coil common-terminal voltage CT is higher than thedetection level, turns the differential signal Co high, and, when themotor coil common-terminal voltage CT is not higher than the detectionlevel, turns the differential signal Co low.

[0047]FIGS. 9A and 9B are time charts showing the waveforms of theshort-period pulse signal T₁ output from the pulse generator 4 and thevoltages observed at relevant points in the circuit shown in FIG. 6before and after the time point t₁. Here, CT₀ represents the voltage atthe node between the resistors R1 to R3, i.e., the virtualcommon-terminal voltage, and LV represents the voltage fed to theinverting input terminal of the comparator 18, i.e., the virtualcommon-terminal voltage having its level shifted. FIG. 9A shows thevoltage waveforms observed when the rotor position before the startingof the motor is “9” (in the range of electrical angles from 240° to270°), and FIG. 9B shows the voltage waveforms observed when the rotorposition before the starting of the motor is “1” (in the range ofelectrical angles from 0° to 30°).

[0048] Now, why the level of the virtual common-terminal voltage isshifted will be described. FIG. 8 shows the relationship between thecommon-terminal voltage CT of the stator coils as detected on thetrailing edges of the short-period pulse signal T₁ with the motorvoltages V_(uout), V_(vout), and V_(wout) applied to the motor 2 duringthe period from t₅ to t₆ shown in FIG. 3 and the electrical angle of therotor position before the starting of the motor. The common-terminalvoltage CT of the stator coils has a small peak around an electricalangle of 45° and a large peak around an electrical angle of 225°. If thedetection level is lower than the small peak around an electrical angleof 45°, it is impossible to correctly detect the rotor position. Forthis reason, the level of the virtual common-terminal voltage is shiftedso that the detection level never becomes lower than the small peakaround an electrical angle of 45°.

[0049] Instead of the configuration shown in FIG. 6, the configurationshown in FIG. 7 may be used. The circuit shown in FIG. 7 differs fromthat shown in FIG. 6 in that the differential amplifier 15 and theresistor R4 are omitted from the circuit shown in FIG. 6 and instead thenode between the resistors R1 to R3 is directly connected to the nodebetween the constant current sources 16 and 17 and the comparator 18.

[0050] The circuits shown in FIGS. 6 and 7 may be additionally providedwith another two serial pairs of constant current sources, each pairhaving two constant current sources connected in series. In that case,each serial pair of constant current sources is so set as to output adifferent current, and a selector circuit for selecting one constantcurrent source from the three pairs of constant current sources isprovided. This makes it possible to shift the level by different amountsin the different periods from t₀ to t₁, from t₁ to t₂, . . . , and fromt₅ to t₆. This increases the number of level shift patterns to six,while there are two in the circuits shown in FIGS. 6 and 7. Advisably,the number of level shift patterns is determined according to the typeof the motor actually used.

[0051] Next, as an example of an electric appliance embodying theinvention, an optical disk playback apparatus will be described withreference to FIG. 10. An optical pickup apparatus 22 irradiates anoptical disk 23, such as a minidisk, with laser light, then reads asignal from the laser light reflected from the optical disk 23, and thenfeeds the read signal to a microcomputer 20.

[0052] A driver circuit 21 drives the optical pickup apparatus 22according to instructions from the microcomputer 20. The optical pickupapparatus 22 is moved, by a stepping motor (not illustrated)incorporated therein, stepwise in the radial direction of the opticaldisk 23 so as to be positioned at the track where to read a signal.

[0053] Moreover, a driver circuit 24 drives a spindle motor 25 accordingto instructions from the microcomputer 20. The optical disk 23 isrotated via a spindle by the spindle motor 25.

[0054] Here, used as the driver circuit 24 and the spindle motor 25 arethe motor drive apparatus shown in FIG. 1. Specifically, as the spindlemotor 25 is used the motor 2, and as the driver circuit 24 is used acircuit composed of the drive portion 1 and the motor drive controlcircuit 3. The motor drive apparatus shown in FIG. 1 never starts themotor in the reverse direction, and therefore never requires an undulylong time to start the spindle motor 25. Moreover, the motor driveapparatus shown in FIG. 1 operates with reduced power consumption, andis therefore suitable for use in a portable optical disk playbackapparatus that operates from a battery.

[0055] Incidentally, detecting the rotor position before the starting ofa motor and then starting the motor according to the detection result aspracticed in the embodiments described above is called “sensing start.”The embodiments described hereinbefore deal with cases where the driveportion 1 and the motor drive control circuit 3 are formed in separateICs. However, it is also possible to form the drive portion 1 and themotor drive control circuit 3 in a single IC.

What is claimed is:
 1. A motor drive control circuit that controls, withdrive signals, a drive portion that feeds drive currents to stator coilsof a motor, comprising: a drive signal output circuit; and a detectioncircuit, wherein the drive signal output circuit outputs rotor positiondetection drive signals to the drive portion before starting of themotor, and wherein the detection circuit receives a common-terminalvoltage of the stator coils when the drive signal output circuit isoutputting the rotor position detection drive signals, and detects,according to the common-terminal voltage of the stator coils, a positionof a rotor before starting of the motor.
 2. A motor drive controlcircuit as claimed in claim 1, wherein the rotor position detectiondrive signals are signals that control the drive portion so that, beforestarting of the motor, the drive portion feeds the stator coils withrotor position detection drive currents that cause the common-terminalvoltage of the stator coils to vary with the position of the rotor butthat do not cause the motor to rotate.
 3. A motor drive control circuitas claimed in claim 1, wherein the detection circuit includes aplurality of resistors, a detection level generation circuit, and acomparator, and detects, according to the output of the comparator, theposition of the rotor before starting of the motor, wherein theplurality of resistors, at one ends thereof, respectively receivevoltages at nodes between the motor and the drive portion and, at theother ends thereof, are connected together, wherein the detection levelgeneration circuit generates detection levels by shifting a voltage atthe other ends of the plurality of resistors to levels corresponding tothe rotor position detection drive signals, and wherein the comparatorcompares the detection levels with the common-terminal voltage of thestator coils.
 4. A motor drive control circuit as claimed in claim 3,wherein the detection level generation circuit includes a serial circuithaving two constant-current sources connected in series, and the otherends of the plurality of resistors are connected, either directly orthrough a buffer and a resistor, to a node between the twoconstant-current sources.
 5. A motor drive apparatus comprising: amotor; a drive portion that feeds drive currents to stator coils of themotor; and a motor drive control circuit that controls the drive portionwith drive signals, wherein the motor drive control circuit includes adrive signal output circuit and a detection circuit, wherein the drivesignal output circuit outputs rotor position detection drive signals tothe drive portion before starting of the motor, and wherein thedetection circuit receives a common-terminal voltage of the stator coilswhen the drive signal output circuit is outputting the rotor positiondetection drive signals, and detects, according to the common-terminalvoltage of the stator coils, a position of a rotor before starting ofthe motor.
 6. A motor drive apparatus as claimed in claim 5, wherein therotor position detection drive signals are signals that control thedrive portion so that, before starting of the motor, the drive portionfeeds the stator coils with rotor position detection drive currents thatcause the common-terminal voltage of the stator coils to vary with theposition of the rotor but that do not cause the motor to rotate.
 7. Amotor drive apparatus as claimed in claim 5, wherein the detectioncircuit includes a plurality of resistors, a detection level generationcircuit, and a comparator, and detects, according to the output of thecomparator, the position of the rotor before starting of the motor,wherein the plurality of resistors, at one ends thereof, respectivelyreceive voltages at nodes between the motor and the drive portion and,at the other ends thereof, are connected together, wherein the detectionlevel generation circuit generates detection levels by shifting avoltage at the other ends of the plurality of resistors to levelscorresponding to the rotor position detection drive signals, and whereinthe comparator compares the detection levels with the common-terminalvoltage of the stator coils.
 8. A motor drive apparatus as claimed inclaim 7, wherein the detection level generation circuit includes aserial circuit having two constant-current sources connected in series,and the other ends of the plurality of resistors are connected, eitherdirectly or through a buffer and a resistor, to a node between the twoconstant-current sources.
 9. An electric appliance comprising: a motordrive apparatus including: a motor; a drive portion that feeds drivecurrents to stator coils of the motor; and a motor drive control circuitthat controls the drive portion with drive signals; and a rotatingmember that is driven by the motor drive apparatus, wherein the motordrive control circuit includes a drive signal output circuit and adetection circuit, wherein the drive signal output circuit outputs rotorposition detection drive signals to the drive portion before starting ofthe motor, and wherein the detection circuit receives a common-terminalvoltage of the stator coils when the drive signal output circuit isoutputting the rotor position detection drive signals, and detects,according to the common-terminal voltage of the stator coils, a positionof a rotor before starting of the motor.
 10. An electric appliance asclaimed in claim 9, wherein the rotor position detection drive signalsare signals that control the drive portion so that, before starting ofthe motor, the drive portion feeds the stator coils with rotor positiondetection drive currents that cause the common-terminal voltage of thestator coils to vary with the position of the rotor but that do notcause the motor to rotate.
 11. An electric appliance as claimed in claim9, wherein the detection circuit includes a plurality of resistors, adetection level generation circuit, and a comparator, and detects,according to the output of the comparator, the position of the rotorbefore starting of the motor, wherein the plurality of resistors, at oneends thereof, respectively receive voltages at nodes between the motorand the drive portion and, at the other ends thereof, are connectedtogether, wherein the detection level generation circuit generatesdetection levels by shifting a voltage at the other ends of theplurality of resistors to levels corresponding to the rotor positiondetection drive signals, and wherein the comparator compares thedetection levels with the common-terminal voltage of the stator coils.12. An electric appliance as claimed in claim 11, wherein the detectionlevel generation circuit includes a serial circuit having twoconstant-current sources connected in series, and the other ends of theplurality of resistors are connected, either directly or through abuffer and a resistor, to a node between the two constant-currentsources.
 13. An electric appliance as claimed in claim 9, wherein theelectric appliance is an optical disk playback apparatus, and the motoris a spindle motor.
 14. An electric appliance as claimed in claim 10,wherein the electric appliance is an optical disk playback apparatus,and the motor is a spindle motor.
 15. An electric appliance as claimedin claim 11, wherein the electric appliance is an optical disk playbackapparatus, and the motor is a spindle motor.
 16. An electric applianceas claimed in claim 12, wherein the electric appliance is an opticaldisk playback apparatus, and the motor is a spindle motor.